1. Field of the Invention
The present invention relates to an electron beam apparatus, such as a scanning electron microscope, electron probe microanalyzer, or Auger spectrometer using a field emission gun as its electron beam source. The invention also relates to a method of controlling such an electron beam apparatus.
2. Description of Related Art
In a scanning electron microscope, an electron beam is sharply focused onto a specimen. A desired range on the specimen is scanned with the beam. The electron beam irradiation of the specimen produces secondary electrons, which arc detected. The resulting signal is supplied to a display, such as a CRT synchronized with the scanning of the primary electron beam. In this way, a scanned image of the specimen is displayed.
In an electron probe microanalyzer, an electron beam is directed at a specimen. Characteristic X-rays emanating from the specimen are detected. In an Auger spectrometer, Auger electrons from a specimen surface are detected, and their energies are measured.
In these various kinds of electron beam apparatus, field emission guns using a wide range of emission currents have been recently used as electron beam sources. In an electron beam apparatus, such as a scanning electron microscope, electron probe microanalyzer, or Auger spectrometer, the emission current needs to be varied over a wide range from the order of pico-amperes to hundreds of nano-amperes.
In order to collect the on-axis electron beam efficiently with a field emission gun using a small amount of total emission current, the first condenser lens is preferably mounted above the anode electrode aperture. It is known that in an electron beam apparatus fitted with a field emission gun (FEG), the resolution at high electron beam (probe) currents of more than nano-amperes deteriorates dependently on the angular current density of the field emission electron source and on the spherical aberration coefficient of the first condenser lens.
Furthermore, the angular current density is determined by the field emission electron source. Therefore, it is necessary to improve the spherical aberration coefficient of the first condenser lens in order to improve the resolution at high electron beam currents. For this reason, in an electron optical system using the conventional two-stage condenser lens system, the amount of electron beam current is controlled by the first condenser lens. The angular aperture conditions are optimized with the second condenser lens.
Where the first condenser lens is placed close to the field emission emitter, the distance between the principal plane of the first condenser lens and the object point is too small. Therefore, it is difficult to use the first condenser lens in the real image mode. Consequently, in the conventional first condenser lens, the electron beam current has been continuously controlled in the virtual image mode. In this case, the aberration coefficient of the first condenser lens determining the resolution at high electron beam currents cannot be taken into consideration. In addition, where the first condenser lens is placed close to the emitter and the apparatus is used in the virtual image mode, even if a two-stage condenser lens system is used, limitations are imposed on the variable range of the electron beam current.
It is an object of the present invention to provide an electron beam apparatus that can be used optimally at any electron beam current.
It is another object of the present invention to provide a method of controlling this electron beam apparatus.
An electron beam apparatus according to the present invention comprises: (a) a field emission gun having a field emission emitter, an extraction electrode for extracting electrons from the emitter to thereby extract an electron beam, and an anode electrode for accelerating the electron beam emitted from the emitter; (b) a first condenser lens for collimating the electron beam extracted by the extraction electrode; (c) a second condenser lens mounted behind the anode electrode; (d) an angular aperture control lens; and (e) an objective lens for focusing the electron beam onto a specimen. The electron beam is collimated or made a real-image mode beam by the first condenser lens.
The present invention also provides a method for controlling an electron beam apparatus having (a) a field emission gun having a field emission emitter, an extraction electrode for extracting electrons from the emitter to thereby extract an electron beam, and an anode electrode for accelerating the electron beam emitted from the emitter; (b) a first condenser lens for collimating the electron beam extracted by the extraction electrode; (c) a second condenser lens mounted behind the anode electrode; (d) an angular aperture control lens; and (e) an objective lens for focusing the electron beam onto a specimen. The method consists of collimating the electron beam or making the beam a real-image mode beam by the first condenser lens.